Tubular Condensers Having Tubes with External Enhancements

ABSTRACT

Improvements in tubes, which increase the heat exchange capacity of tubular heat exchangers using the tubes, are described. These improvements involve the use of one or more external surface enhancements, optionally combined with an internal enhancement and/or differing tube geometries. These improvements apply, for example, to internal condensers, including those in which the tube bundles are oriented vertically, in vapor-liquid contacting apparatuses such as distillation columns.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of copending application Ser. No.12/433,064 which was filed on Apr. 30, 2009, the contents of which areincorporated herein by reference thereto.

FIELD OF THE INVENTION

The invention relates to tubes that are generally contained in tubebundles and have enhanced capacity for exchanging heat between fluidsexternal to the tubes and passing through the tubes. An exemplary tubebundle extends vertically within a distillation column and is used tocondense liquid from vapors generated in the column.

DESCRIPTION OF RELATED ART

Heat exchangers are prevalent in refining, petrochemical, and otherindustrial applications in order to efficiently transfer heat availablein one process fluid to another fluid, such that the overall utilityrequirements are reduced. The advantages of using heat exchangers, forexample, to optimize the recovery of heat and thereby minimize the costsassociated with outside sources of cooling/refrigeration media (e.g.,cooling water) and/or heating media (e.g., fuel gas) are wellrecognized. The art has continually sought to improve the performance ofheat exchangers by achieving the closest possible approach to theequilibrium level of heat transfer between two fluid streams at thelowest possible equipment, operating, and maintenance costs.

Heat exchange is commonly carried out, for example, between a relativelyhot reactor effluent fluid and a relatively cold reactor feed fluid. Acombined reactor feed/effluent heat exchanger in this case canbeneficially add at least a portion of the heat required to raise thereactor feed to a specified reaction temperature and at the same timeremove at least a portion of the heat required to cool the reactoreffluent for further processing or storage. A specific application forheat exchangers involves their use within (rather than external to)other processing equipment such as vapor-liquid contacting apparatusesand even reactors. In the case of a reactor, for example, a heatexchanger within a vapor space above a reaction zone may beneficiallycondense evaporated reactants while allowing the removal of uncondensedvapors, particularly product vapors.

Vapor-liquid contacting apparatuses known to utilize internal heatexchangers, and particularly condensers, include distillation columns.The specific operating conditions of distillation columns employinginternal heat exchangers may vary significantly in order to accomplish awide range of component separations from vastly different types ofmixtures that may be subjected to distillation. Examples of distillationcolumns include those used in a number of column separations such asstripping and rectification, as well as those used in various forms ofdistillation such as fractional distillation, steam distillation,reactive distillation, and distillation in divided wall columns. Theseseparation processes may be operated using distillation columns ineither batch or continuous modes, with common design objectives beingthe reduction in installed and operating costs. The equipment andutilities required for the supply and removal of heat to and from thecolumn significantly impact these costs in many cases.

Various benefits may be achieved from installing heat exchangers insidedistillation columns or other apparatuses, rather than external to thecylindrical column shell. These benefits may be appreciated withreference to the operation of conventional external heat exchangers,which require removing a stream from the column, passing it through theexternal exchanger to supply or remove heat, and returning at least partof the heated or cooled stream back to the column. For example, overheadvapor may be withdrawn from a top or overhead section of the column(e.g., after rising from a top contacting tray) and passed to anexternal heat exchanger, namely a condenser or partial condenser, thatcondenses liquid from the withdrawn vapor, a portion of which is thengenerally returned (e.g., using a pump) to the column as reflux. Inaddition to an external heat exchanger and pump, the overhead systemfrequently also comprises a receiver vessel to separate the condensedliquid from uncondensed vapor, as well as the associated pipes, valves,and instrumentation. In a manner analogous to that of a condenser,external reboiler heat exchangers may also provide vapor to (rather thanremove vapor from) the column by heating a liquid stream removed fromthe bottom section of the column. Likewise, vapor and liquid streams maybe withdrawn from a central section between the top and bottom sectionsof the column, heated or cooled using an external heat exchanger, andreturned to the column. In each case, the equipment requirements arecomparable.

By locating a heat exchanger within a vapor-liquid contacting apparatussuch as a distillation column, some equipment (e.g., an overhead refluxpump) and the associated supporting structure can be eliminated, therebysaving both equipment cost and space. Additionally, the pressure dropthrough an internal heat exchanger can be lower than that for anequivalent external heat exchange system. This becomes an importantfactor when the column is operated at nearly atmospheric or evensub-atmospheric pressure, for example in cases where the columntemperatures are limited due to heat sensitivity of the mixture beingprocessed in the column.

Representative examples of low pressure distillation columns in whichinternal condensers have been successfully employed include those usedin the product recovery sections in the commercial production of phenolvia cumene oxidation, as well as in the upstream production of cumenevia benzene alkylation. Also, U.S. Pat. No. 2,044,372, U.S. Pat. No.4,218,289, U.S. Pat. No. 5,507,356, and DE 198 30 163 A1 describe theuse of various heat exchangers inside columns to at least partiallycondense vapor in the top section of columns. U.S. Pat. No. 2,044,372describes the use of a vertical submerged condenser between a lowpressure section and a high pressure section of a single column.

A particular type of heat exchanger that may be used internally (or thatis otherwise commonly employed commercially), is a tubular exchangercomprising a bundle of tubes, whereby heat is transferred between fluidexternal to the tubes and fluid passing through the tubes. So-called“stabbed-in” tube bundles have advantages over internal welded platebundles in terms of their ease of removal for maintenance orreplacement. In the case of stabbed-in tubular condensers, with thetubes being oriented horizontally or vertically in the top section of adistillation column, overhead vapor in the column is condensed on theoutside or external surface of the tubes.

The operation of such internal tubular condensers, however, is normallyassociated with low pressure drops and/or low local mass velocities ofthe fluid being cooled (e.g., column overhead vapor that is enriched ina lower boiling component). This results in low heat transfercoefficients that can translate, particularly in the case of largecolumns, to required heat transfer surface areas that exceed what ispractically installed (in terms of size and/or weight) as a tube bundle.Conventional approaches in these situations have been to resort to theuse of internal, welded plate heat exchangers or even externalcondensers. There is therefore an ongoing need in the art forimprovements in the heat transfer coefficient (and the correspondingreduction in required condenser tube surface area), and/or overallperformance of tubular condensers, and particularly those disposedwithin a vapor-liquid contacting apparatus such as a distillationcolumn.

SUMMARY OF THE INVENTION

The present invention is associated with the discovery of improvementsin heat exchangers comprising tube bundles and particularly internaltubular condensers disposed within vapor-liquid contacting apparatusessuch as distillation columns. Aspects of the invention relate tocondenser tubes having surface enhancements that improve theirperformance, especially when aligned or extending substantiallyvertically within a section (e.g., an overhead section) of the length ofa vertically oriented column. The surface enhancements of the tubesbeneficially improve their heat transfer coefficient and consequentlythe overall heat exchange capacity of an internal tubular condenserbundle of a given size that employs these tubes. This higher capacity insome instances (e.g., in the case of large columns and/or columnsoperating in low pressure drop/low mass velocity regimes) can overcomethe requirement to use more costly heat exchangers such as welded plateor external heat exchangers. In particular, the tube surfaceenhancements described herein can increase the heat transfer coefficientof tubes used in a tubular condenser such that the required exchangerarea is reduced to below that which corresponds to a practical sizelimit (e.g., about 1.5 meters (5 feet) diameter of the tube bundle) orweight limit for installation at the top of a distillation column.

Embodiments of the invention are directed to apparatuses forvapor-liquid contacting. Representative apparatuses include distillationcolumns as well as reactors, including those used in reactivedistillation. Other types of reactors are those which may benefit frominternally condensing at least a portion of vapors within the reactor.For example, it may be desired to condense a condensable portion of thereactor effluent within the reactor to provide an internal reflux and/oravoid all or at least part of the downstream cooling requirements.

The apparatuses comprise a vertical or substantially vertical column(e.g., a cylindrical column having an axis that is aligned vertically orwithin about 5 degrees of vertical). The column contains or has disposedtherein a plurality of condenser tubes or a tube bundle (e.g., in theform of a U-tube bundle with U-shaped individual tubes) of the internaltubular condenser. According to particular embodiments, the tubes extendsubstantially horizontally or otherwise substantially vertically over asection of the column length, for example an overhead section near thetop of the column. All or at least a portion of the condenser tubes haveexternal surfaces comprising one or more surface enhancements to improvethe heat transfer coefficient of the tubes.

Representative surface enhancements include shaped recessions,circumferentially extending fins, axially extending fins, or acombination of these. In the case of circumferentially extending fins,the fins may be characteristic of those used for “low finned” tubes,with the fins having a height from about 0.76 mm (0.03 inches) to about3.8 mm (0.15 inches). Circumferentially extending fins generally referto a plurality of “plates” that are spaced apart (e.g., uniformly or atregular intervals) along the axial direction of the tube. The plates ofcircumferentially extending fins, in an alternative embodiment, may beprovided by a single, continuously wound, helical spiral rather thandiscreet extensions. In either case, the plates often each have an outeredge (or outer perimeter), with a single tube extending through centralsections of a plurality of plates. The outer edges of the plates may becircular or may have some other geometry, such as rectangular orelliptical. In the case of circumferentially extending fins, furthertube surface enhancements can include one or more notches on the outeredges of all or a portion of these fins or plates, where the notches maybe spaced apart radially about the edges, for example, in a uniformmanner or at a constant radial spacing. In other embodiments,non-uniform radial spacing may be used. In the case of tubes used in avertically aligned tube bundle of a condenser, it may be desirable toalign the notches axially with respect to adjacent fins (i.e., theimmediately higher and/or lower circumferentially extending fins). Theaxial alignment of these notches, such that they may be superimposedwhen viewed axially, can improve condensate drainage.

In the case of shaped recessions on the tube surface, all or at least aportion of the recessions may extend axially (e.g., in the form of oneor more elongated troughs) or otherwise be aligned in one or moreaxially extending rows (e.g., in the form of a plurality of discreet,smaller recessions). One or more axially extending fins may also be usedas a tube surface enhancement to improve the heat transfer coefficientof the tubes. Combinations of any of the surface enhancements describedherein are generally all located in the same region of the tubes usedfor heat transfer, for example the region extending substantiallyvertically over a section of the length of a distillation column. Thesurface enhancements may also be combined with other features such as atwisted tube geometry, as discussed below, in this region. In aparticular embodiment, for example, tubes having a twisted tube geometrymay also have circumferential fins as surface enhancements. In a morespecific embodiment, these circumferential fins can have outer edgesthat include a plurality notches. In yet more specific embodiments, thenotches may be aligned axially with respect to adjacentcircumferentially extending fins and/or they may be bent at theirrespective corners outside of the plane of the circumferentiallyextending fins.

Alone or in combination with surface enhancements, the tubes themselves,while extending in a generally linear direction, may have, in at leastone region of the tubes used for heat transfer as described above, anon-linear central axis, which can provide a non-linear internal flowpath for fluid flow through the tubes. For example, the tubes, as wellas their internal central axes, may have a wave, jagged, or helical(coiled) shape to increase pressure drop and/or fluid mixing. Otherwise,an overall helical fluid flow path can be provided, for example, in thecase of a flattened or eccentric profile tube (e.g., having arectangular cross-section or otherwise an oval-shaped or ellipticalcross section) that has a twisted tube geometry (i.e., such that a majoraxis of the cross-sectional shape, for example the major axis of anellipse, rotates clockwise or counterclockwise along the lineardirection of the tube). In the case of a twisted tube geometry, thecentral axis of fluid flow may be linear or non-linear (e.g., helical).Adjacent tubes extending generally linearly, for example in adistillation column section where heat transfer takes place, but havinga wave, jagged, or helical shape or a twisted tube geometry may have aplurality of external contact points with adjacent tubes, with thesecontact points possibly being evenly spaced apart by regions where theadjacent tubes are not in contact. Such spaced apart contact points withone or more adjacent tubes can physically stabilize the positions of thetubes and even avoid the need for baffles or tube supports.

Alternatively, an enhanced condensing layer (ECL) may be applied to theoutside or external surfaces of the tubular condenser tubes as anothertype of surface enhancement. Examples of ECLs include textured surfaces,chemical coatings that improve drop-wise condensation, nano-coatings,etc.

In addition to their exterior surfaces, the tube internal surfaces maybe modified to improve heat transfer capability. For example, all, amajority, or at least a portion of the tubes in the tube bundle may haveinternal surfaces, at least in a region of the tubes that extends (e.g.,vertically or horizontally) over a section of the column length, ontowhich a coating is bonded. If a coating is used, it is generally bondedto at least a region of the tubes (e.g., where condensation occurs onthe external tube surfaces) having the surface enhancement(s), asdiscussed above, on outer or external surfaces. A representativeinternal tube surface coating comprises a porous metallic matrix thatcan improve the internal heat transfer coefficient of the tube andconsequently the overall heat exchange capacity of a condenser using thetubes. Some suitable coatings are referred to as enhanced boiling layers(EBLs), which are known in the art for their applicability to heattransfer surfaces on which boiling occurs, and particularly for theirability to achieve a high degree of heat transfer at relatively lowtemperature differences. An EBL often has a structure comprising amultitude of pores that provide boiling nucleation sites to facilitateboiling.

An EBL or other coating may be applied to the inside or internalsurfaces of tubular condenser tubes. A representative metal coating isapplied as described, for example, in U.S. Pat. No. 3,384,154. Thecoated metal is subjected to a reducing atmosphere and heated to atemperature for sufficient time so that the metal particles sinter orbraze together and to the base metal surface. An EBL may also havemechanically or chemically formed reentrant grooves as described, forexample, in U.S. Pat. No. 3,457,990. Other known methods of applyingcoatings and EBLs in particular to metal surfaces, such as the internalsurfaces of metal tubes, that may be used include those described in GB2 034 355, U.S. Pat. No. 4,258,783, GB 2 062 207, EP 303 493, U.S. Pat.No. 4,767,497, U.S. Pat. No. 4,846,267, and EP 112 782.

In addition to EBLs, another internal enhancement for condenser tubesinvolves the use of one or a plurality of ridges, which may, forexample, be in the form of a spiral or multiple spirals. Such ridges maybe used to further improve the transfer of heat, and particularlysensible heat, across the internal tube surface. Internal ridges may beused alone or in combination with other features of condenser tubes asdescribed herein. Further internal enhancements include twisted tape,wire matrix inserts (e.g., from Cal-Gavin Limited, Warwickshire, UK),and other in-tube heat transfer devices that can enhance the tubesideheat transfer coefficient.

Other embodiments of the invention are directed to tube bundles for acondenser comprising tubes as described above. According to particularembodiments, at least a portion of the tubes, in an axially (e.g.,vertically or horizontally, depending on the orientation of the tubebundle) extending section, have internal surfaces having a coating(e.g., a porous metallic matrix as discussed above) bonded thereon, andhave external surfaces comprising circumferentially extending fins.These circumferential fins may be characteristic of “low finned” tubeshaving a height from about 0.76 mm (0.03 inches) to about 3.8 mm (0.15inches). The circumferentially extending fins may have outer edges thatinclude a plurality notches. According to particular embodiments in thecase of notches being present on the circumferentially extending fins,these notches may be aligned axially with respect to adjacentcircumferentially extending fins and/or they may be bent at theirrespective corners outside of the plane of the circumferentiallyextending fins. According to other embodiments, at least a portion ofthe tubes have a non-linear central axis and/or a twisted geometry, asdiscussed above, in the axially extending section, and optionally aplurality of spaced apart points of external contact with adjacenttubes. According to other embodiments, tube bundles may comprise tubeshaving two or more types of shaped recessions on the external surface,namely smaller discreet shaped recessions and larger, axially extendedshaped recessions. The smaller recessions can advantageously providecapillary action to reduce the condensate fluid layer thickness by usingthe liquid surface tension. Outer edges of these recessions may bealigned in axially extending rows along the external surfaces of thetubes.

Further aspects of the invention relate to the use of any of thecondenser tubes as described above in tube bundles for heat exchangeapplications. Accordingly, embodiments of the invention are directed tomethods of indirectly exchanging heat between two fluids, a first fluidand a second fluid, comprising contacting the first fluid with externalsurfaces of any of the condenser tubes as described above and passingthe second fluid through the tubes. In particular applications in whichthe tube bundles are used as condensers, the first fluid is hotter thanthe second fluid, and a fraction of this first fluid is condensed onexternal surfaces of the tube bundle.

A particular heat exchange application of commercial interest involvesthe use of the condenser tubes in internal condensers for vapor-liquidcontacting apparatuses such as distillation columns. Distillation refersto a separation process based on differences in the relative volatilityof components present in an impure mixture. Distillation involves thepurification of components having differing relative volatilities byachieving multiple theoretical stages of vapor-liquid equilibrium alongthe length of a vertical column. Rising vapor, enriched in a lowerboiling component relative to the liquid from which it is vaporized in alower stage in the column, is contacted with falling liquid, enriched ina higher boiling component relative to the vapor from which it iscondensed in a higher stage in the column.

Accordingly, particular embodiments of the invention are directed to theuse of tube bundles, comprising tubes as described above, in heatexchangers within vapor-liquid contacting apparatuses such asdistillation columns. A first fluid comprising distillation columnvapor, often rising in a top or overhead section of the column, isenriched in a lower boiling component, the purification of which is theobjective of the distillation. This first fluid is contacted with theexternal surfaces of the tubes or tube bundle (e.g., to condense aliquid from this first fluid that is enriched in the lower boilingcomponent, relative to the impure mixture being fed into the column andpurified), while a second, cooling fluid (e.g., cooling water) is passedthrough the tubes. As a result of contacting the first fluid with theexternal surfaces of the tubes, a condensed liquid and a non-condensedvapor are formed. The non-condensed vapor may be removed from thecolumn, while the condensed liquid is returned to an immediately lowercontacting stage, for example as overhead reflux. These and otheraspects and embodiments associated with the present invention areapparent from the following Detailed Description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the upper section of a distillation column having aninternal tubular condenser, with the tubes extending vertically over asection of the column length.

FIG. 2 depicts a representative section of a tube for a tubularcondenser, in which the tube has external surface enhancements in theform of circumferentially extending fins as discreet extensions.

FIG. 2A depicts a representative section of a tube for a tubularcondenser, in which the tube has external surface enhancements in theform of circumferentially extending fins as a single, continuouslywound, helical spiral.

FIG. 3 depicts a cross-sectional view of the tube section of FIG. 2,through A-A′.

FIG. 4A depicts a modification of the tube of FIGS. 2 and 3, in which aplurality of axially aligned notches, having a curved cross-sectionalshape, are included on outer edges of circumferentially extending fins.

FIG. 4B depicts a further modification of the tube of FIGS. 5 and 6, inwhich a plurality of axially aligned notches, having a triangularcross-sectional shape, are included on outer edges of circumferentiallyextending fins.

FIG. 4C depicts a representative section of a tube havingcircumferentially extending fins as shown in FIG. 4B, but with thenotches being bent at their respective corners outside of the plane ofthe circumferentially extending fins, and in opposite directions.

FIG. 5 depicts a representative section of a tube having surfaceenhancements in the form of small shaped recessions aligned in axiallyextending rows that alternate, about the radial tube periphery, withlarger, axially extending shaped recessions in the form of troughs, aswell as internal spiral ridges.

FIG. 6 depicts a representative section of a tube having surfaceenhancements in the form of axially extending shaped recessions ortroughs that form axially extending ridges resulting from the axialextension of sections or points of the external tube surface that do notform the recessions.

FIG. 7A depicts a cross-sectional view of a tube having surfaceenhancements in the form of axially extending shaped recessions having asemi-circular cross-sectional shape and spaced about the radial tubeperiphery.

FIG. 7B depicts a cross-sectional view of a tube having surfaceenhancements in the form of axially extending shaped recessions having atriangular cross-sectional shape and spaced about the radial tubeperiphery.

FIG. 7C depicts a cross-sectional view of a tube having surfaceenhancements including both axially extending shaped recessions, havinga notched, triangular cross-sectional shape, that form alternatingaxially extending ridges.

FIG. 7D depicts a cross-sectional view of a tube having surfaceenhancements in the form of axially extending shaped recessions having asemi-circular cross-sectional shape, with the axially extending ridgesformed between these shaped recessions also having a semi-circularcross-sectional shape.

FIG. 8 depicts a representative section of a tube having surfaceenhancements in the form of axially extending fins that are spaced aboutthe radial tube periphery.

FIG. 9 depicts a cross-sectional view of the tube section of FIG. 8,through A-A′.

FIG. 10A depicts a representative section of a tube having a twistedtube geometry as a tube enhancement.

FIG. 10B depicts a cross-sectional view of the tube of FIG. 10A.

The same reference numbers are used to illustrate the same or similarfeatures throughout the drawings. The drawings are to be understood topresent an illustration of the invention and/or principles involved. Asis readily apparent to one of skill in the art having knowledge of thepresent disclosure, vapor-liquid contacting apparatuses, andparticularly those comprising vertically oriented columns having tubularcondensers disposed therein, according to various other embodiments ofthe invention, will have configurations (e.g., a number of tube passes)and components determined, in part, by their specific use.

DETAILED DESCRIPTION

The invention is associated with improvements in heat exchangers andparticularly internal tubular condensers used in vapor-liquid contactingapparatuses such as distillation columns. Internal tubular condensers,often referred to in the art as “column installed” or “stabbed-in”tubular condensers when used to condense vapors generated indistillation, are typically installed in the upper vapor-liquidcontacting section of a column, either vertically from the top, as shownin FIG. 1, or horizontally from the side. As discussed above, suchinternal tubular condensers are normally desirable in low massvelocity/low pressure drop services, but their economic attractivenesscompared to external heat exchangers or internal, welded plateexchangers is often dictated by their performance in terms of the heattransfer coefficient of the individual tubes and consequently theoverall heat exchange capacity of the condenser. The use of tubes havingone or more surface enhancements, particularly in the case of verticallyextending internal tubular condensers can effectively improve theirperformance.

Vertically extending condensers refer in particular to heat exchangershaving a tube bundle in which the tubes extend vertically orsubstantially vertically over a section (e.g., a top or overheadsection) of the column length. Condensate forming on these tubes musttherefore drain vertically, or along at least a portion of the axiallength of the tubes. The various tube surface enhancements describedherein may serve, alone or in combination, to facilitate this condensatedrainage and/or reduce the layer thickness of formed condensate, therebyimproving the heat transfer coefficient of the tubes. In representativeembodiments, for example, the use of such surface enhancement(s) willgenerally increase the tube heat transfer coefficient in a givencondensing service (e.g., in a distillation column used in the productrecovery section in the commercial production of phenol via cumeneoxidation) by a factor of at least about 1.5, typically from about 2 toabout 10, and often from about 3 to about 5, relative to the heattransfer coefficient obtained with identical tubes but lacking thesurface enhancement(s).

As discussed above, this improvement in heat transfer coefficientdecreases the tube area needed, such that tubular condensers employingthese enhancements can be feasibly installed in larger-diameterdistillation columns, for example those having a diameter of generallygreater than about 0.9 meters (3 feet), typically in the range fromabout 1.07 meters (3.5 feet) to about 6.10 meters (20 feet), and oftenin the range from about 1.22 (4 feet) to about 4.88 meters (16 feet).The use of tube bundles in tubular condensers, in which at least aportion of the individual tubes have surface enhancements as describedherein, may in some cases provide an economically attractivealternative, relative to external condensers or even welded plateinternal condensers. Any of the tubes described below, having surfaceenhancements, will generally have an outer diameter in the range fromabout 13 mm (0.5 inches) to about 38 mm (1.5 inches), and often fromabout 19 mm (0.75 inches) to about 32 mm (1.25 inches). The innerdiameters of such tubes are generally in the range from about 6 mm (0.25inches) to about 32 mm (1.25 inches), and often from about 13 mm (0.5inches) to about 25 mm (1 inch). The inner and outer diameters can bedetermined and/or optimized for a given service based on a number offactors, including the design flow rates, pressure drops, and heattransfer coefficients, as will be appreciated by those having skill inthe art and knowledge of the present disclosure.

FIG. 1 shows an upper section of a distillation column 20 having aninternal tubular condenser 30, with a plurality of tubes 2 extendingvertically over a section 4 of the column length. It is recognized thatthe entire length of the tube will generally not extend only in onedirection, but will normally curve, for example in a U-bend, as shown inFIG. 1, to redirect fluid passing through the tubes back to a commontube sheet 6 securing the two ends of each of the tubes so that they arein communication with respective tube-side inlet 8 and outlet 10conduits. During operation, upwardly flowing vapor 12 a in the uppersection of the distillation column 20 contacts tubes 2, through whichcooling fluid (e.g., cooling water) is passed from the tube-side inlet 8to the tube-side outlet 10. Contact between the relatively hot vapor 12a, comprising condensable material, and the relatively cool externalsurfaces of tubes 2 causes condensation, with condensed liquid fallingback into the column interior and non-condensed vapor exiting through anon-condensed vapor outlet 14 that may be in communication with thecolumn exterior. Both (i) the upwardly flowing vapor 12 a and (ii) thefraction of this upwardly flowing vapor 12 a that is the non-condensedvapor exiting the column are enriched, as a result of the distillation,in a lower boiling component initially present in an impure mixture.Compared to the upwardly flowing vapor 12 a within the column, thenon-condensed vapor exiting the column through outlet 14 will normallybe more enriched in this component, as a result of removing additional,higher boiling impurities through condensation. To improve contactingwith tubes 2, the flow of upwardly-flowing vapor 12 a may be diverted(e.g., in a side-to-side manner as it travels generally upwardly throughtube bundle) using one or more baffles 16.

In the case of internal distillation column condensers, and particularlythose having vertically or substantially vertically oriented condensertubes, surface enhancements, in at least the region of the tubesextending over a section of the column length, include circumferentiallyextending fins, as illustrated in FIG. 2. FIG. 2A showscircumferentially extending fins 15 a provided by a single, continuouslywound, helical spiral rather than discreet extensions, as shown in FIG.2. In the case of tubes 2 comprising circumferentially extending fins 15a, a fin height of less than about 6.4 mm (0.25 inches) isrepresentative, with fin heights typically being in the range from about0.51 mm (0.02 inches) to about 5.1 mm (0.20 inches), and often being inthe range from about 0.76 mm (0.03 inches) to about 3.8 mm (0.15inches). As is illustrated in the cross-sectional view of FIG. 3,circumferentially extending fins 15 a may be in the form of flat platesor discs having a circular cross section that is concentric withcircular cross sections of internal surface 25 and external surface 27with these cross sections being circles with inner and outer diameters,respectively, of tubes 2. The fin height can therefore be measured asthe distance from the external surface 27 of a tube 2 to the outer edge29 of circumferentially extending fin 15 a. In cases where the fins havegeometries that are not circular (e.g., elliptical or rectangular),where the fin cross sectional shape is not concentric with the centralaxis of the tube 2, or where the tube 2 itself has a non-circular (e.g.,flattened or elliptical) cross section, the fin height may be theaverage distance from the outer edge 29 of circumferentially extendingfin 15 a to the external surface 27 of tube 2.

FIG. 4A shows a cross-sectional view of a tube 2 having surfaceenhancements in the form of fins 15 a, as shown in FIGS. 2 and 3. In theembodiment illustrated in FIG. 4A, however, a plurality of notches 35are “cut” from, or shaped in, the outer edges 29 of fins 15 a. Notches35 shown in FIG. 4A have a curved cross-sectional shape (e.g.,semi-circular), but other curved cross-sectional shapes or rectangularcross-sectional shapes may be used for notches 35. For example, FIG. 4Bshows notches 35 having a triangular cross-sectional shape. Also asillustrated in FIGS. 4A and 4B, notches may be spaced evenly about theouter edge 29 or periphery of fin 15 a. In a particular embodiment, inwhich circumferentially extending fins 15 a, as surface enhancements,have outer edges 29 that include notches 35 having a triangular (orother) cross-sectional shape, these notches 35 may be bent at theirrespective corners 37 outside of the plane of the circumferentiallyextending fins, for example opposing corners 37 of a triangular crosssection may be bent in the same or opposite directions. In theparticular embodiment illustrated in FIG. 4C, for example, these notches35 are bent at their respective corners 37 in opposite directions. In apreferred embodiment, and particularly in the case in which the tubesare used in vertical, column-installed condensers, all or a portion ofnotches 35, whether or not they are bent, may be aligned axially, withone or more corresponding notch(es) in the outer edge of one or bothadjacent circumferentially extending fins (e.g., in both of thecircumferentially extending fins located immediately above andimmediately below, in the case of a vertically extending tube). Axialalignment of notches is also illustrated in the representativeembodiment of FIG. 4C. This axial alignment of notches can promoteimproved drainage of condensate from the tubes, particularly in thevertical direction.

In the same manner as described above with respect to notches on outeredges of fins, notches or recessions having various cross-sectionalshapes may be formed directly on the outer surfaces of heat exchangertubes to provide surface enhancements. Extending these notches in theaxial direction on the tube surface results in elongated troughs aboutthe tube periphery. Alternatively, discreet, shaped recessions may beformed on the external tube surface. While the recessions themselves maybe small, if desired, in order to provide an effective capillary actionthat reduces condensate layer thickness, such smaller recessions may bealigned axially to provide an axial or generally axial flow path forcondensed liquid. FIG. 5 depicts tubes 2 having shaped recessions 36 a,36 b on the external surface, where a portion of these recessions 36 aare smaller and are aligned in axially extending rows 22 a, for example,with outer edges of the recessions in a row 22 a forming a line thatextends axially along the external surface of the tube. As discussedabove, these smaller, discreet shaped recessions 36 a on the tubesurface can act as capillaries, such that the surface tension of thecondensed liquid is drawn into recessions 36 a. In a representativeembodiment, in order to provide capillary action, each individual shapedrecession will normally have only a small area, typically less thanabout 5 mm2 (7.8×10-3 in2) and often in the range from about 0.1 mm2(1.6×10-4 in2) to about 4 mm2 (6.2×10-3 in2). Aligning at least some ofthe recessions in one or more axially extending rows allows thecondensed liquid to effectively drain vertically, for example, in avertically extending internal tubular condenser. In the embodiment shownin FIG. 5, the axially aligned, smaller, discreet shaped recessions 36 aare used as surface enhancements in combination with axially elongatedshaped recessions 36 b (i.e., with the individual recessions extendingover a longer axial portion). Both of these surface enhancements may beused in a common region of the tube that extends over a section of thelength of a distillation column where condensation occurs. In theparticular embodiment illustrated in FIG. 5, rows 22 a of discreet,shaped recessions 36 a alternate radially about the tube periphery withrows 22 b of larger, axially extending shaped recessions 36 b (e.g., inthe form of troughs), between which rows the external surface 27 of tube2 may be smooth. FIG. 5 also depicts internal enhancements on internalsurface 25, namely spiral ridges 21, which may be used for improved heatexchange. In FIG. 6, the axially extending, shaped recessions 36 b arein the form of troughs having a triangular cross-sectional shape.

FIGS. 7A-7D illustrate in more detail some representative cross sectionsof tubes having shaped recessions 36 on their external surfaces 27. Inparticular, the shaped recessions 36 in FIGS. 7A and 7D have a curvedcross-sectional shape that is semi-circular, while the shaped recessions36 in FIG. 7B have a triangular cross-sectional shape. Other curved andrectangular (e.g., semi-elliptical and square) cross-sectional shapesare possible. Another embodiment in which tube surfaces are enhancedwith shaped recessions 36 is shown in FIG. 7C, where, as in FIG. 7B, thecross-sectional shapes of recessions 36, spaced (e.g., uniformly) aboutthe periphery of the surface of tube 2, are triangles. In the embodimentshown in FIG. 7C, however, these triangles are broad enough such thatonly small sections or points of the external surface 27 of tube 2remain (or are not part of the shaped recessions), with these sectionsbeing spaced radially about the periphery of tube 2. The axial extensionof these sections or points results in axially extending ridges. Such atube with axially extending, shaped recessions 36 b or troughs alignedin axial rows 22 b is also illustrated in the front view of FIG. 6.

In FIG. 7D, the axially extending ridges, similarly formed between theseshaped recessions, have a smooth, curved (e.g., semi-circular)cross-sectional shape of the same or a similar dimension as the curvedcross sectional shape forming the shaped recessions. The cross sectionalshape of this tube therefore has a generally circular perimeter definedby alternating, concave and convex curves (e.g., semi-circles). Theresulting, smooth external surface contrasts with the embodiment shownin FIG. 7A, where the shaped recessions form edges. Therefore, as shown,for example in the embodiment of FIG. 7D, the shaped recessions canprovide a fluted profile of a fluted tube. Fluted tubes or other tubeshaving axially extending shaped recessions or discreet, shapedrecessions aligned in axially extending rows as depicted, for example,in FIGS. 7A-7D may be characterized as having two outer diameters.Smaller and larger outer diameters may be the distances, respectively,to opposing deepest points of recessions 36 and opposing externalsurfaces 27, with each of these distances being measured through thecenter of the cross section of tube 2. Representative tubes havingaxially extending shaped recessions will have smaller and larger outerdiameters in the ranges from about 13 mm (0.5 inches) to about 32 mm(1.25 inches) and from about 19 mm (0.75 inches) to about 38 mm (1.5inches), respectively. In exemplary embodiments, such a tube will haveouter diameters of about 19 mm (0.75 inches) and about 25 mm (1.0inches) or outer diameters of about 25 mm (1.0 inches) and about 32 mm(1.25 inches).

Additional surface enhancements to improve heat transfer for verticallyextending tubes are shown in FIG. 8, which depicts tubes having aplurality of axially extending fins 15 b that may, for example, be inthe form of flat plates raised above the external surface 27 of the tube2 and extending axially along the length of the tube. Representativefins may have a fin height as described above with respect to theheights of circumferentially extending fins, with the fin height alsobeing based on the distance (or average distance) between the outer edge29 of axially extending fin 15 b and the external surface 27 of tube 2.Otherwise, the fin heights of axially extending fins may be relativelyhigher, for example with ranges from about 3.2 mm (0.125 inches) toabout 25 mm (1 inch), and often from about 6.4 mm (0.50 inches) to about19 mm (0.75 inches) being representative.

A cross-sectional view of the tube shown in FIG. 8, having a pluralityof axially extending fins 15 b, in this case spaced uniformly about theradial periphery of the tube 2, is shown in FIG. 9. As discussed abovewith respect to circumferentially extending fins (15 a in FIG. 2),axially extending fins 15 b may also have notches with variouscross-sectional shapes. FIG. 10A illustrates a tube having a twistedtube geometry to provide an overall helical fluid flow path within thetube. As seen in the cross-sectional view of FIG. 10B the eccentricprofile tube has an oval-shaped cross section 50, with the major axis 55that rotates clockwise or counterclockwise along the linear direction ofthe tube).

Any of the axially extending features (i.e., in the same orsubstantially the same direction as the central axis of the tube)discussed above, such as axially extending shaped recessions, axiallyextending rows of shaped recessions, or axially extending fins, aretherefore vertically or horizontally extending features, depending onwhether the tubes are aligned vertically or horizontally, respectively.In alternative embodiments, any of the described, axially extendingfeatures may extend or be aligned generally in the axial direction alongthe length of the external surface of the tube, in a non-linear pathsuch as a wave, spiral, jagged line, etc. Such embodiments provide agenerally axial flow path (e.g., corresponding to the downward flow pathof condensed liquid along the tube when positioned vertically) for fluidcontacting the heat exchange surface, where this flow path provided bythe features is not directly, but only generally, axially.

The use of axially or generally extending shaped recessions and/or fins,in this manner, as tube surface enhancements, can reduce condensate filmthickness and/or facilitate condensate drainage, thereby improving theheat transfer coefficient of the tube. Such features as surfaceenhancements for tubes are particularly advantageous in internal tubularcondensers (e.g., disposed in distillation columns), where the heatexchange surface area, as well as the total weight of equipment that canbe practically installed (e.g., at or near the top of the column ortower) are both limited. The tube surface enhancements discussed abovemay be used alone or in combination. The tube surface enhancements mayalso be used in combination with internal enhancements as discussedabove, and particularly spiral ridges that may act to further improveheat transfer. Otherwise, these surface enhancements may be combinedwith a coating, such as a porous metallic matrix used to form anenhanced boiling layer as discussed above, that is bonded onto internalsurfaces of the tubes, for example, in at least the same region of thetubes (e.g., extending over a section of the column height) as thesurface enhancements. The surface enhancements may also be used in tubebundles in which all or a portion of the tubes have a non-linear centralaxis (e.g., a helical axis), or otherwise have a twisted tube geometryas discussed above, in at least the same region of the tubes as thesurface enhancements. In a representative embodiment, for example, atube bundle of a condenser, having tubes with a fluted tube profile andan internal enhancement including one or more spiral ridges, is alignedvertically in the upper section of a distillation column. Various othercombinations of surface enhancements, optionally with an internalsurface coating and/or non-linear or twisted geometries, can beincorporated into tubes to improve their heat transfer coefficient,particularly when the tubes are used in a tube bundle that is orientedvertically and used in a service in which condensate drains verticallyfrom the external surfaces of the tubes (i.e., on the “shell side” ofthe condenser).

Overall, aspects of the invention are directed to improvements in heatexchangers and particularly tubular exchangers oriented horizontally orvertically within contacting apparatuses such as distillation columns.Those having skill in the art, with the knowledge gained from thepresent disclosure, will recognize that various changes can be made inthe above apparatuses, heat exchangers, tubes, and vapor-liquidcontacting (e.g., distillation) methods without departing from the scopeof the present disclosure. Mechanisms used to explain theoretical orobserved phenomena or results, shall be interpreted as illustrative onlyand not limiting in any way the scope of the appended claims.

1. A method of purifying a lower boiling component from an impuremixture by distillation, the method comprising: (a) contacting a vaporenriched in the lower boiling component on the external surfaces of aplurality of condenser tubes of an apparatus for vapor-liquidcontacting, comprising a vertically oriented column having disposedtherein the plurality of condenser tubes extending substantiallyvertically over a section of the column length, wherein at least aportion of the condenser tubes have external surfaces comprising one ormore surface enhancements, and (b) passing a cooling fluid through thecondenser tubes.
 2. The method of claim 1, wherein the surfaceenhancements are selected from the group consisting of shapedrecessions, circumferentially extending fins, axially extending fins, anexternal condensing layer, and combinations thereof.
 3. The method ofclaim 2, wherein the surface enhancements comprise shaped recessions andat least a portion of the shaped recessions extend axially or arealigned in one or more axially extending rows.
 4. The method of claim 2,wherein the surface enhancements comprise circumferentially extendingfins.
 5. The method of claim 4, wherein the circumferentially extendingfins have a height from about 0.76 mm (0.03 inches) to about 3.8 mm(0.15 inches).
 6. The method of claim 4, wherein at least a portion ofthe circumferentially extending fins have outer edges that include aplurality notches aligned axially with respect to adjacentcircumferentially extending fins.
 7. The method of claim 6, wherein thenotches are bent at their respective corners outside of the plane of thecircumferentially extending fins.
 8. The method of claim 3, wherein theshaped recessions comprise both smaller, discreet shaped recessions andlarger, axially extended shaped recessions.
 9. The method of claim 2,the portion of tubes further comprise an internal enhancement thatincludes one or more spiral ridges.
 10. The method of claim 1, whereinthe condenser tubes have a twisted tube geometry in a region of thetubes that extends over the section of the column length.
 11. The methodof claim 1, wherein at least a portion of the condenser tubes haveinternal surfaces having an internal enhancement in a region of thetubes that extends over the section of the column length.
 12. The methodof claim 11, wherein the internal enhancement is a coating comprising aporous metallic matrix.
 13. The method of claim 11, wherein the internalenhancement includes one or more spiral ridges.
 14. A method ofexchanging heat between a first fluid and a second fluid, the methodcomprising condensing the first fluid on external surfaces of a tubebundle for a condenser comprising tubes, wherein at least a portion ofthe tubes, in an axially extending section, have internal surfaceshaving a coating comprising a porous metallic matrix bonded thereon, andhave external surfaces comprising circumferentially extending fins, andpassing the second fluid through the tubes.
 15. The method of claim 14,wherein the first fluid is enriched in a lower boiling component that ispurified in a distillation column comprising the tubes.
 16. The methodof claim 14, wherein the circumferentially extending fins have a heightfrom about 0.76 mm (0.03 inches) to about 3.8 mm (0.15 inches).
 17. Themethod of claim 14, wherein the circumferentially extending fins haveouter edges that include a plurality notches.
 18. The method of claim17, wherein the notches are bent at their respective corners outside ofthe plane of the circumferentially extending fins.